1. Field of the Invention
The present invention is directed to a high powered laser mirror and the method of making the same, said mirrors special utility with respect to carbon dioxide gas lasers operating at a wavelength of 10.6 microns, where continuous power densities in excess of about 30 kilowatts per square centimeter are expected. As typical examples, such lasers can be employed in sealing pressurized nuclear fuel elements and may find use in the future for the generation of power by means of laser induced fusion.
2. Description of the Prior Art
Additionally, the invention also relates to laser mirrors for use in the ultraviolet regions especially in the range between about 500A and about 1000A and such mirrors are characterized by exceedingly low light scatter of the order of about 1% and less.
The mirror which is employed in either continuous or pulsed laser system, is highly critical in determining the efficiency. The duration of laser pulses is improved by use of such mirror. This results from the fact that heating is caused by absorbed laser energy which in turn is responsible for degradation and destruction of the laser mirror because the heat causes distortion. Ideally, the laser should have low optical absorptivity at the laser wavelength and high thermal conductivity to dissipate the energy which is absorbed by the mirror. Evaporated films of silver, gold, and copper have very low absorptivity in the visible and near infrared spectral regions and it has been found that the absorptivity of the film is a function of the purity of the metal and the roughness of the substrate upon which the film is deposited. The absorptivity of the laser energy arising from surface roughness has been termed the "anomalous skin effect." The micro-roughness of the film surface can cause diffuse scattering of the conduction electrons at the surface thereby giving rise to an increase in the absorptivity which can be as large as 50% of the absorptivity for a smooth surface. Prior art laser mirrors have absorptivities of about 1% when utilizing evaporated films of silver or gold deposited on copper substrates.
It has also been found that with high power laser mirrors efficiency is destroyed by reason of a lowered figure of merit which has been assigned to the prior art mirrors. The figure of merit appears to be an all encompassing value but is particularly susceptible to differential thermal expansion caused by non-uniform laser heating. Apparently, prior art mirrors in which copper or molybdenum are employed as the substrates appear to be particularly susceptible which is believed due in part to the high value for the thermal expansion coefficient of copper or molybdenum.